The present invention relates to improvements in electric motors of the type including a commutator and brushes, and more particularly to a structure for such electric motors which permits simple assembling and reliable operation of the electric motors, and a method of manufacturing components therefore. Electric motors according to the present invention are very useful as compact electric motors to be used for power tools or the like.
In conventional assembling of an electric motor including a commutator and brushes, wiring of leads to brushes or stator coils is intricate and, at the same time, apt to cause trouble, so that various techniques have been proposed to eliminate the problems. Such techniques include, for example:
a technique of providing a terminal board to be attached to an end face of a stator to simplify connection between stator coils and leads;
a technique of employing a brush assembly including brushes and a circuit to be connected with the brushes so as to simplify connection between the brushes and the leads; and
a technique which is a combination of the above two techniques and employs both of a terminal board and a brush assembly.
These techniques are disclosed, for example, in U.S. Pat. Nos. 3,924,147, 3,979,615, 4,038,573, 4,056,749, 4,071,793, 4,370,579, 4,593,220 and 4,673,837, and Japanese Laid-Open Patent Publication No. 59-113763 and Japanese Laid-Open Utility Model Publication No. 60-138338.
Though the aforementioned techniques have generally been successful in simplifying the manufacture of electric motors of the type including a commutator and brushes, they did have certain drawbacks and disadvantages.
In the technique of employing a conventional brush assembly, two types of brush assemblies are required to manufacture electric motors which are rotatable in both forward and reverse directions, which causes reduction of efficiency of mass production and troublesome storage of brush assemblies.
In a conventional terminal board, sufficient consideration is not given to the connection between the terminal board and ends of stator coils.
For example, the above listed U.S. Pat. Nos. 3,979,615 and 4,038,573 disclose a structure which connects stator coils with a terminal board. The connecting structure includes a box-like contact retaining element or cavity of a terminal board mounted on an end face of a stator, and an end of the stator coils is inserted through a wire inlet of the cavity. Thereafter, when a contact element or terminal is fitted into the cavity, the end is caught in a groove formed in the terminal, and at this time, a portion of the outer insulating jacket of the end is removed by contact with the groove surface to electrically connect the terminal with the end. However, the end can sometimes float, depending on the position of the cavity into which the terminal connected with the end is fitted. Such a floating end of the stator coils may be broken under continuous vibration during operation of a power tool or the like. This must be avoided by all means.
There has not been yet proposed a compact electric motor of the type including a stator having field coils and brake coils in which all connections of the electric motor can be achieved by means of a single brush assembly and a single terminal board connected thereto. In order to achieve such a structure, the brush assembly must incorporate a connecting circuit for brushes, a connecting circuit for field coils, a connecting circuit for brake coils and connecting means for connecting the brushes, field coils and brake coils. Such a brush assembly will become large in size and impossible to be used for a compact motor.
For example, the above listed Japanese Laid-Open Patent Publication No. 59-113763 and Japanese Laid-Open Utility Model Publication No. 60-138338 disclose the structure of an electric motor having field coils and brake coils. This structure includes two terminal boards, one for the field coils and the other for the brake coils disposed on respective sides of the stator.
In the above prior art commutator type electric motor, terminals connected to the ends of the stator coils are provided on both end faces of the stator, so that wiring is required from the respective terminals on both end faces of the stator to the brush assembly and a starting switch or power switch through leads. Space for such wiring must be provided and the leads must be laid in a stable manner. Furthermore, as the brush assembly carrying brushes and other components is to be fixedly assembled in a housing, for example, of a power tool, assembly of such a commutator type electric motor becomes a time-consuming job, thus reducing the productivity of power tools or the like.